Electric steering system with mechanical back-up device

ABSTRACT

A steering system for an automotive vehicle, includes a steering device to be operated by a driver, a rotatable connection having a drive end connected to the steering device and a drive end connectable to a linkage of the steering wheels of the vehicle. The drive end of the rotatable connection is provided with a first sensor, the driven end of the rotatable connection is provided with a second sensor and an electric motor, the first and second sensors being connected to a control device, the control device controlling the electric motor on the basis of signals emanating from the first and second sensors.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention is related to a steering system for an automotive vehicle,comprising a steering device to be operated by a driver, a rotatableconnection having a drive end connected to the steering device and adriven end connectable to a linkage of the steering wheels of thevehicle.

2. Description of Related Art

Such a steering system is known, and usually comprises a purelymechanical rotatable connection, e.g. a tubular shaft. Said shaft isalso provided with an absorber mechanism for protecting the driver incase of a frontal collision.

Although such prior art steering system performs satisfactorily, itnevertheless has disadvantages as well. The short tubular shaft stillmay present a potential hazard to the driver. Furthermore, itsapplication in the surrounding vehicle construction imposes limitations,on the vehicle design as well. Its routing from the steering wheel tothe linkage is rather straightforward, and can hardly be varied orrouted along a different path.

Usually, also a power assist device is connected to the steering systemso as to alleviate the steering torque to be exerted by the driver onthe steering wheel. Although generally hydraulic power assist devicesare applied in this connection, electric power assist devices have beenproposed as well. Such electric power assist devices have the advantagethat no hydraulics are needed. The hydraulic oil, which presentsenvironmental disadvantages, and the somewhat cumbersome lay out ofhydraulic connections can then be dispensed with.

SUMMARY OF THE INVENTION

The object of the invention is to provide a steering system which can beapplied in a more versatile way, and which does not have thedisadvantages of the prior art hydraulically assisted steering devices.This object is achieved in that the drive end of the rotatableconnection is provided with a first sensor, the driven end of therotatable connection is provided with a second sensor and an electricmotor, the first and second sensors being connected to a control device,said control device controlling the electric motor on the basis ofsignals emanating from the first and second sensors.

The electric motor can drive the driven end of the rotatable connectionin several ways. First of all, the electric motor can act as a powerassist device, analogous to the hydraulic assist devices. The rotatableconnection in that case still delivers some torque for steeringpurposes. However, it is also possible to provide the full steeringtorque through the electric motor. In that case, the rotatableconnection merely acts as a mechanical backup of the fully electricsteering system in the case of e.g. an electric power failure.

Preferably, the first sensor is incorporated in a first bearing and thesecond sensor is incorporated in a second bearing, said bearings beingconnected to the rotatable connection for supporting said rotatableconnection with respect to the vehicle. Such sensor bearings constitutestandard available, proven components, and can easily and reliably beincorporated in the steering system.

In case the rotatable connection can be carried out as a mechanicalbackup drive, it can be a flexible torque cable, e.g. a Bowden cable.Such cable can be adapted more easily to the space constraints within avehicle. Nevertheless, such cable may also be combined with a shaftportion.

Furthermore, the drive end of the rotatable connection can be providedwith an accelerating gear drive and the driven end of the rotatableconnection can be provided with a decelerating gear drive.Alternatively, a standard steering column can be used as a mechanicalback-up drive.

As a result of the gear drives in question, the rotatable connection hasa relatively high rotational speed. Thereby, it becomes possible toapply a rotatable connection, such as a Bowden cable or shaft, withrelatively small cross-sectional dimensions without impairing thetorsional stiffness of the steering system.

Moreover, a relatively high number of revolutions of the rotatableconnection makes it possible to apply relatively simple sensors withonly small resolution. The relatively high number of resolutions whichexhibits the rotatable connection makes that such simple, economicsensors still can provide the required angular accuracy.

The sensors are positioned between the accelerating and the deceleratinggear drives. The combined ratio can be selected to fit the steeringdesign, but is often suitably selected as 1.

The electric motor can be positioned between the accelerating and thedecelerating gear drives as well, other positions however are alsopossible, e.g. at the decelerating gear. Thus, an electric motor withoutgear reduction can be applied directly to the rotatable connection andto the linkage.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The invention will now be described further with reference to anembodiment shown in the FIGURE.

The FIGURE shows a schematic lay out of an electric steering system 1according to the invention, which is connected to the wheel 2 of avehicle through a mechanical linkage 3 which is known per se and whichwill therefore not be described further.

The steering system 1 comprises a steering wheel 4 as well as arotatable connection 5 which extends between the steering wheel 4 andthe linkage 3. This rotatable connection 5 can for instance be carriedout as a so-called Bowden cable, that is a flexible cable with astationary hollow outer tubular cable, in combination with arotationally stiff inner cable accommodated within the outer cable. SuchBowden cables are known per se as well, and will therefore not bedetailed further. This rotatable connection can also be carried out as ashaft.

At its drive end 6 the rotatable connection 5 is connected to anaccelerating gear drive (accelerating gearbox 7), which in turn isconnected to the steering wheel 4.

At its driven end 8 the rotatable connection 5 is connected to adecelerating gear drive (decelerating gearbox 9), which in turn inconnected to the linkage 3. The combined gear ratio of the acceleratinggearbox 7 and the decelerating gearbox 9 can be 1; other ratios arepossible as well.

At the output end of the accelerating gear box 7, a sensor bearing 10 isprovided, and a similar sensor bearing 11 is provided at the input endof the decelerating gear box 9. These sensor bearings 10, 11 areconnected to a control unit 12.

Upon rotating the steering wheel 4, the sensor bearing 10 senses theaccelerated rotation at the output end of the accelerating gearbox 7.The control unit 12 receives signals from this sensor bearing 10 andalso from the sensor bearing 11 at the input end of the deceleratinggearbox 9. In case differences are measured, the control unit throughline 15 delivers a control signal to the electric motor 16 so as toprovide the desired rotational speed at the input end of thedecelerating gear box 9, which of course depends on the rotations of thesteering wheel 4.

Preferably, the electric steering motor 16 provides the full rotationaltorque required driving the linkage system. In that case, the rotatableconnection 5 functions merely as a mechanical back up, which would takeover the steering control in case of an electric power failure.

The steering device is connected to a force feedback motor 16 so as tosimulate the forces which are exerted on the vehicle wheels duringdriving. This force feedback motor is controlled through line 17 by thecontrol unit 12. The force feedback motor 18 obtains a signal related tothe power which is needed to drive the steering motor 16. The more poweris delivered by the steering motor 16 for a steering action of thewheels 2, the more torque is then transferred to the steering wheel 4 bythe force feedback motor 18.

1. Steering system for an automotive vehicle, comprising a steering device to be operated by a driver, a rotatable connection having a drive end connected to the steering device and a driven end connectable to a linkage of the steering wheels of the vehicle, wherein the drive end of the rotatable connection is provided with a first sensor, the driven end of the rotatable connection is provided with a second sensor and an electric steering motor, the first and second sensors being connected to a control device controlling the electric steering motor on the basis of signal emanating from the first and second sensors, characterized in that the drive end of the rotatable connection is provided with an accelerating gear drive and in that the driven end of the rotatable connection is provided with a decelerating gear drive, the gear drives each having a rotational input member and a rotational output member.
 2. Steering system according to claim 1, wherein the first sensor is incorporated in a first bearing and the second sensor is incorporated in a second bearing, said bearings being connected to the rotatable connection for supporting said rotatable connection with respect to the vehicle.
 3. Steering system according to claim 2, wherein the rotatable connection comprises a flexible torque cable such as a Bowden cable.
 4. Steering system according to claim 2, wherein the rotatable connection comprises a shaft.
 5. Steering system according to claim 1, wherein the sensors are positioned between the accelerating and the decelerating gear drives.
 6. Steering system according to claim 1, wherein the combined gear ratio of the accelerating and of the decelerating gear drives is about
 1. 7. Steering system according to claim 6, wherein the electric motor is positioned between the accelerating and decelerating gear drives.
 8. Steering system according to claim 1, wherein the steering device is connected to a force feedback motor.
 9. Steering system according to claim 8, wherein the force feedback motor is controllable by means of the control device.
 10. Steering system according to claim 8, wherein the force feedback motor is positioned between the accelerating and decelerating gear drives. 